1. Field of the Invention
This invention relates to stress-crack resistant ethylene-perhaloethylene polymers, particularly polymers of about 40 to 60 mol percent of ethylene with from about 40 to 60 mol percent of chlorotrifluoroethylene, tetrafluoroethylene or mixtures thereof and to a process for their preparation.
2. Description of the Prior Art
Copolymers of about 40 to about 60 mol percent of ethylene with correspondingly from about 60 to about 40 mol percent chlorotrifluoroethylene, tetrafluoroethylene or mixtures thereof are known in the art and are disclosed, for example, in U.S. Pat. No. 3,847,881 issued in 1974 to Mueller et al. While such copolymers have high melting points, generally in the order of about about 200.degree. C., their use at temperatures above about 150.degree. C. has been severely restricted due to their tendency to thermal stress-crack at such temperatures. At such temperatures, the copolymers exhibit cracks under tensile stresses appreciably below the limits of their short-term strength, especially when powder coated onto metal substrates.
To overcome the thermal stress-cracking tendency of such copolymers, it has been suggested in the aforementioned copending application to provide such copolymers with high and low molecular weight components in specified amounts. Although such modified copolymers exhibit high resistance to thermal stress-cracking, it would be desirable to further improve the high temperature properties of such copolymers without adversely affecting other desirable properties.